Mark bucket as condition_unknown properly

Previously, if the condition was unknown for the entire bucket, the
bucket would be annotated with a NO_DATA rather than being dropped
because CONDITION_UNKNOWN. This updates all the buckets to be dropped
due to CONDITION_UNKNOWN. Note that this will make the condition unknown
problem appear worse, when in reality it is making our bucket drop
reasons more correct and should not have any impact on the true
CONDITION_UNKNOWN.

Also fix an issue where a test was failing due to a previous test
leaving data persisted to disk. This fix is a hack for now, and should
be properly fixed later.

Bug: 158897179
Test: atest statsd_test
Change-Id: Ic703600d37aa1873ec67cb1a279dcf349d48fc9f